Distributors in special regions including a city, an industrial area and a commercial area generally use low voltage networks of 600 volts or less. Specially, cables of the networks are laid under the ground, which are designed to inflow into it at sites more than one. Such cables may suffer from faults caused by a thermal degradation, an aging, humidity or animals such as rats or squirrels. A circuit breaker is provided in order to protect the networks from the causes. The circuit breaker is required to have cutting units such as fuses in order to isolate the cable having a fault and minimize network faults in both ends of a cable. The cable cutting units can safely operate in a phase-to-phase fault such as a high voltage and low impedance fault.
Generally, a miniature circuit breaker and an earth leakage breaker are used in the home to protect a fire or an electric shock accident. The miniature circuit breaker is used to protect cables and its operations are as follows. Firstly, in case that a load current is over the rating current, the current flowing in the circuit breaker is higher than normal current and it causes heat. This heat makes an inner Bimetal bent and cuts off an operation of electrical equipment. Secondly, in case that a phase-to-phase short circuit occurs in the load side by an electric tool or other metal material, an high voltage is generated instantaneously and then the Bimetal is heated. So, the inner magnet starts to operate and cuts off the electric equipment before the operation of it. The high voltage generates a lot of magnetic field and then operates the inner magnet of the electric equipment. In case of the earth leakage breaker, there is provided a function that a user can be protected by detecting and cutting off power when the user is struck by electricity in using the electrical equipment, in addition to a function of the miniature circuit breaker.
In United States, it is required that the distributor has the miniature circuit breaker and a consent directly touched by hands of the user has a ground fault circuit interrupter (GFCI). The ground fault circuit interrupter (GFCI) being a kind of earth leakage breaker has a high sensitive function of detecting an electric leakage and it is compulsory to use the interrupter in places having high humidity such as a kitchen, a both room, a parking lot.
Even though the miniature circuit breakers and the earth leakage breakers are established and used, many fires broke out all over the world every year. This is because an arcing type fault to the ground is frequently occurred rather than the phase-to-phase fault described above. Since this arc fault is of a low current and a high impedance and generates a current having a root mean square (RMS) less than a thermal threshold of the breaker, the cable cutting apparatus may not respond to the fault and therefore a fire breaks out in many cases.
Nevertheless, the arc fault is very dangerous since it occurs in a high temperature. And the arc fault can be detected by a Ground Fault Circuit Interrupter (GFCI) in the only case that the arc fault generates a sufficient leakage current through a ground. Moreover, since the interrupter operates when the current occurred by the arc exceeds a parameter of a thermal/magnetic structure of the breaker, an Arc Fault Circuit Interrupter (AFCI) for breaking the arc fault is necessarily required. Specially, Consumer Product Safety Commission (CPSC) gave a decision that 40% of fires broken out in 1977 were due to the arc faults. Accordingly, the National Electric Code (NEC) imposed duty upon every home to use the arc fault circuit interrupter (AFCI) from January of 2002. Causes of arc fault are very various, for example, an aging, breakdowns of insulation and wire, a mechanical and electrical stress by an excessive usage or an excessive voltage, an connection fault, and an excessive mechanical fault to wires. Generally, arc faults occurred in a residential building or a commercial building can be classified into three cases.
Firstly, there is a serial arc (contact arc) occurred between wires serially connected to the load. FIG. 1 shows a case where a serial arc has been occurred. Referring to FIG. 1, a wire 14 and 16 constituting a cable 10 is separated and covered by an insulator 12 so that it can be insulated. In FIG. 1, an upper wire 14 is broken in a predetermined area and a serial gap 18 is generated. When an arc occurs in this state, a lot of heat occurs in the cable locally. And when the heat continues to occur enough to break or carbonize an insulator adjacent to the arc occurring area, a fire would break out. In the serial arc, a magnitude of a current flowing in the arc is controlled by the load.
Secondly, there is a parallel arc (line arc) occurred between conductive wires, which is drawn in FIG. 2. Referring to FIG. 2, conductive wires 24 and 26 in a cable 20 are surrounded by an outer insulator 22 and insulated by an inner insulator 28. When the inner insulator 28 is aged or injured like the part 21, an arc fault 23 occurs between an upper conductive wire 24 and a lower conductive wire 26. The aging or injury of the inner insulator can be generated by a carbonization occurred by excessive exposure of direct ray of light or the lightning which has an influence on a wire system, or by mechanical operations occurred by the cut of a cable extension code part when the cable is pressed under furniture such as a chair.
Thirdly, there is a ground arc occurred between a conductive wire and the ground, which is drawn in FIG. 3. Referring to FIG. 3, the ground arc occurs when an insulator 38 of a cable 30 protecting conductive wires 34 and 36 like the parallel arc is broken and the conductive wire 36 is grounded through the broken part 39.
Specially, since the parallel arc and the ground arc occur in parallel with a load, a current flowing in the arc changes by an impedance of the power. When the aging phenomenon of the cable continues for a long time as described above, the cover of the cable is damaged due to the carbonization of the cable, and the joule heating is generated due to the arc current so that the cable is aged more severely. At that time, the joule heating is J=(arc current)2×time and an arc occurs due to the carbonization of the cable in accordance with the joule heating.
FIG. 4 is a block diagram showing a constitution of a general apparatus for detecting an arc fault in the art.
Referring to FIG. 4, the conventional apparatus for detecting an art fault includes current detector 400, a signal transformer 402, a level determination unit 404, an arc signal detector 406, and a circuit breaker 408. In the conventional apparatus for detecting an arc fault, the detector 400 detects a current flowing on a phase conductive wire 416, and the signal transformer 402 transforms a signal detected in the current detector to a signal suitable for determining the arc.
The level determination unit 404 determines whether an output level of the signal transformer 402 exceeds a predetermined reference voltage, and generates an output signal when the output level exceeds the reference voltage. The arc signal detector 406 integrates an output signal of the level determination unit 404 and determines whether the integrated signal exceeds a predetermined reference voltage, and generates an arc detection signal when the integrated signal exceeds the reference voltage. The arc detection signal is inputted to the circuit breaker 408, and the circuit breaker breaks the phase conductive wire connecting a source 410 to a load 412.
Referring to the arc detector, the most difficult problem is a signal when starting electric equipment and a signal from a dimmer. Both signals are nearly similar in their forms so that a conventional arc fault detector misconceives the signals occurred when starting the electrical equipment and occurred by the dimmer as an arc and often breaks the circuit.
Since a waveform occurred when starting the electrical equipment and that of a harmful arc occurring a fire and so on are similar, the conventional apparatus for detecting an arc fault discriminates both waveforms by making use of a characteristic that both waveforms have different duty cycles, and breaks the circuit. The reason why the arc signal detector 406 integrates the output signal of the level determination unit is to determine a duty cycle of a signal.
A signal generated when the dimmer is operated is similar with that of an arc in their waveforms and also has a characteristic that its duty cycle is long, which is different from the signal generated when starting the electric equipment. Accordingly, the conventional apparatus for detecting an arc fault has a problem that it misconceives a signal occurred when operating the dimmer as an arc signal and breaks a fixed circuit. Also, since the signal occurred when operating the dimmer is nearly similar with that of an arc signal in their magnitude, an apparatus for detecting the arc using a magnitude of the conventional signal cannot make a distinction between a harmful arc and a signal occurred when operating the dimmer.